1. Field of the Invention
The present invention relates to an exhaust gas purification system for cleaning or purifying the exhaust gas of an internal combustion engine.
2. Description of the Related Art
There has hitherto been disclosed a technique in which in order to clean or purify an exhaust gas emitted from an internal combustion engine, in particular a lean-burn one in which a lean air fuel mixture is burnt, particulate matter contained in the exhaust gas is collected by a filter, which carries thereon a catalyst having the function of oxidizing the substances contained in the exhaust gas, such as, for example, an NOx storage reduction catalyst (hereinafter simply referred to as “NOx catalyst”), and nitrogen oxides (NOx) in the exhaust gas are cleaned or purified at a high purification rate. However, the NOx catalyst has the property of occluding SOx in the exhaust gas in addition to NOx therein, and hence the exhaust gas cleaning or purifying function of the NOx catalyst deteriorates as the amount of the SOx occluded in the NOx catalyst increases.
Here, it is necessary to cause the SOx occluded in the NOx catalyst to be released from the NOx catalyst so as to recover the exhaust gas purification function of the NOx catalyst. In view of such a situation, there has been disclosed a technique in which the NOx occluded in an NOx catalyst is reduced and at the same time the SOx occluded in the NOx catalyst is released by controlling the air-fuel ratio of an exhaust gas flowing into the NOx catalyst to be a rich-side air-fuel ratio (for example, see a first patent document: Japanese patent application laid-open No. H9-53442). In this technique, however, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is controlled to be a rich-side air-fuel ratio regardless of the loaded condition of an associated internal combustion engine, so there is a fear that the temperature of the NOx catalyst might rise excessively or the HC in the exhaust gas might be released into the atmosphere in such cases as when the engine speed is high or when the engine torque is high.
In addition, in order to release the SOx occluded in the NOx catalyst, it is necessary to raise the temperature of the NOx catalyst to a relatively high temperature (e.g., 650° C.) and to control the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to be a rich-side air-fuel ratio. Accordingly, there has also been disclosed a technique in which the particulate matter collected in the filter carried by the NOx catalyst is first oxidized for removal thereof, and it is then easily raised to a temperature necessary for the release of the SOx by making use of the heat generated at that time (for example, see second patent documents: Japanese patent Nos. 2727906 and 2001-271634, Japanese patent application laid-open Nos. H11-107809 and 2002-38932).
The exhaust gas purification ability of the exhaust gas purification device gradually deteriorates in accordance with the increasing amount of SOx in the exhaust gas being occluded in the exhaust gas purification device such as an NOx catalyst, etc., in the exhaust gas purification system. Accordingly, there is a need to cause the SOx occluded in the exhaust gas purification device to be released therefrom thereby to recover the exhaust gas purification ability of the exhaust gas purification device by controlling the temperature of the exhaust gas purification device and the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification device.
To this end, however, it is necessary to raise the temperature of the exhaust gas purification device to a predetermined temperature, and to control the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification device to be a predetermined rich-side air-fuel ratio, but there is a fear that it might be difficult to control the temperature of the exhaust gas purification device and the air-fuel ratio of the exhaust gas in this manner depending upon the loaded condition of the internal combustion engine. Furthermore, there is another fear that the HC in the exhaust gas might be released into the outside air due to the air-fuel ratio of the exhaust gas being controlled to the rich-side air-fuel ratio. Therefore, opportunities for the release of the SOx occluded in the exhaust gas purification device are limited depending upon the loaded condition of the internal combustion engine, thus making it difficult to quickly recover the exhaust gas purification ability of the exhaust gas purification device.